Color xerography

ABSTRACT

A xerographic process in which an object is sequentially exposed through red, green, and blue filters and the resulting electrostatic images developed respectively with cyan, magenta, and yellow toners. Undesirable toner absorbances in other than their complementary colors are compensated for by selective removal or back transfer of toner from paper.

United States Patent [191 Lehman Sept. 17, 1974 COLOR XEROGRAPHY Primar Examiner-John M. Horan 751 t:RihdF.Lh y

I 1 men or c at 6 Falrpon N Y Attorney, Agent, or FirmRobert J. Bird [73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: Aug. 29, 1973 [571 ABSTRACT [21] Appl. No.: 392,580 A xerographic process in which an object is sequentially exposed through red, green, and blue filters and the resulting electrostatic images developed respec- [fi] CCl]. 355/615 323%; tively with cyan, magenta and yellow toners. Undesir d able toner absorbances in other than their comple- 1e 0 earc mentary colors are compensated for by Selective References Cited moval or back transfer of toner from paper.

UNITED STATES PATENTS 4 Cl i 4 D i Fi 3,583,806 6/l97l Bixby 355/4 PATENIEBSEP 1 71924 SHEET 1 [1F 2 1 COLOR XEROGRAPHY BACKGROUND OF THE INVENTION This invention relates to color xerography in general, and more specifically to a method of compensating for undesirable color absorbance in the various toners used in the process.

In the subtractive color system used in color xerography, exposures are made sequentially through red, green, and blue separation filters, and the electrostatic images resulting from these exposures are developed respectively with cyan, magenta, and yellow toners. Cyan, being the complement of red, absorbs primarily red. However, cyan toner also absorbs substantially in the green spectral region and even to some extent in the blue. Similarly, while magenta toner ideally absorbs green light only, it also absorbs substantially in the blue region. Yellow toners are the closest to ideal in that they absorb primarily in the blue region. Due to the variance from the ideal absorbance of these toners, images resulting from the application of multiple toners may have excessive color densities, particularly in the blue and green.

SUMMARY OF THE INVENTION It is'an object of the present invention to provide a xerographic color process in which toner color densities varying from the ideal are modified on a final copy sheet to produce improved copy color fidelity.

Briefly, this invention is practiced in one form by a xerographic process in which a color exposure is followed by development and transfer of a complementary color image. This sequence is repeated for each of the primary colors. At appropriate intervals, toner is back-transferred from the copy paper to the photoreceptor to effect a retrieval therefrom of a predetermined amount of unwanted toner.

For a better understanding of this invention, reference is made to the followingdetailed description given in connection with the accompanying drawing.

DRAWING FIG. 1 is a somewhat schematic representation of a mechanized xerographic color process.

FIG. 2 is a representation of the red separation step in a color xerographic process.

FIG. 3 is a representation, similar to FIG. 2, of the green separation step.

FIG. 4 is a representation, similar to FIG. 2, of the blue separation step.

DESCRIPTION Referring now to FIG. 1, a xerographic color apparatus is shown more or less schematically and is generally indicated at 2. A platen or document holder 4 is shown from its underside disposed adjacent a pair of illuminating lamps 6. A photosensitive surface, such as a xerographic belt or photoreceptor 8 is mounted for movement around a plurality of support and drive rollers 10. A corona charging apparatus 12 and a corona erasing or f cleaning apparatus 14 are suitably mounted adjacent to the photoreceptor surface. A paper drive and image transfer roll 16 and an image fixing or fusing apparatus 18 also form a part of the apparatus 2 for the introduction of copy paper and the fixing of the final image thereon.

The foregoing elements are only representative of well-known components in prior art xerographic apparatus, and their details do not form a part of this invention. Accordingly, further description of their structure will be given only to the extent necessary to an understanding of this invention.

A projection lens 20 is positioned on an optical axis 22 between the platen 4 and the photoreceptor 8 for the purpose of projecting an image of an object 38 at the surface 4 onto the photoreceptor belt 8. A filter wheel or battery 24 is suitably mounted between the platen 4 and photoreceptor 8, and has mounted thereon a plurality of light filters, including a red filter 26, a green filter 28, and a blue filter 30, selectively positionable in the optical axis 22. Toners in colors complementary to the filter colors are provided in reservoirs mounted adjacent the photoreceptor. For example, reservoir 32 contains cyan toner, reservoir 34 contains magenta toner, and reservoir 36 contains yellow toner.

Referring now to FIG. 2, a first exposure step, the red separation, is represented with white light reflecting from an object or document 38, through a red filter 26 and onto the photoreceptor 8. A latent electrostatic image of the characters on object 38, (or, more properly, that portion of the original reflecting blue or green light) appears on the photoreceptor 8, and this latent image is developed with cyan toner and finally transferred to white paper shown at 40.

A simlar exposure, the green separation, is then performed through a green filter 28. The electrostatic image from this exposure (of the blue and red from the original) is developed with magenta toner and then transferred to the surface of paper 40 in registry with the first image.

A third exposure, the blue separation, is performed through a blue filter 30. The electrostatic image from this exposure (of the green and red from the original) is developed with yellow toner and transferred to the surface of paper 40, again in registry with the first and second images.

For the purpose of illustrating the present invention, assume that a document is to be copied upon which the characters are of a neutral gray having a density of 0.70. The cyan toner used for the reproduction has the following relative densities in the indicated spectral-regrons:

Blue .l5

Green .50

Red 1.0

The magenta toner has the following densities:

Blue .40

Green l.0

Red

The yellow toner has the following densities:

Blue I Green Red Red .70

Blue .28

Red

Green .70

Since the densities are additive, it will be apparent that the image of cyan toner, absorbing mostly red but also absorbing heavily in the green region, and magenta toner absorbing primarily green, results in the addition of 0.35 to the desired 0.70 green density. Thus, there is too much green density.

The excess green density can be removed by backtransferring half of the magenta image (in which the green density is 0.70) to the photoreceptor. This is accomplished by performing a second exposure through the red filter and permitting the photoreceptor and its undeveloped electrostatic image of the red separation to roll in contact with the paper and the magenta image thereon. The second red exposure or red separation is an electrostatic record (as is the first) of the cyan toner for it is only from the areas where cyan is imaged that any superimposed magenta is to be retrieved. The charge voltage and transfer bias voltage on the corotron l2 and transfer roll 16, respectively, may be adjusted for this back-transfer step so that the correct amount of magenta toner is back-transferred from paper to photoreceptor. Such an adjustment, once made, can be'automatically switched in and out for the back-transfer step as the machine continuously runs through alternate or successive print and back-transfer cycles.

This same procedure can be extended to masking for the unwanted blue densities of the cyan and magenta toners. Since both toners (cyan and magenta) have approximately the same ratio of blue to green density, i.e., 0.3 and 0.4, respectively, satisfactory compensation can be achieved by using the green filter for masking the blue separation and back-transferring yellow toner.

charging a xerographic photoreceptor with electrostatic charge, projecting an electrostatic image of an original object onto said photoreceptor by successive exposures through color filters, developing said electrostatic image by application thereto of successive toners in color complementary to the respective filters, applying an electrostatic transfer charge relative to a final image support means and said photoreceptor,

transferring said developed image to said support means, repeating said charging step,

. 4 repeating a; projection step through a color filter to recreatean electrostatic image of the original on the photoreceptor,

placing said photoreceptor and the undeveloped electrostatic image thereon in operative position relative to said support means and in registry with the developed image thereon, adjusting the application of said electrostatic transfer charge to effect the removal from said support means of some of the tonermaterial.

2. A xerographic color process as defined in claim 1 wherein said color filters are red, green, and blue and said complementary toners are cyan, magenta, and yellow, respectively.

3. A xerographic color process including the steps of:

a. charging a xerographic photoreceptor with electrostatic charge,

b. projecting an electrostatic i'rnage'of an original object onto said photoreceptor by exposure through a red color filter,

c. developing said electrostatic image by application thereto of cyan toner,

d. applying an electrostatic transfer charge relative to a final support means and said photoreceptor,

e. transferring said developed image to said support means,-

f. repeating steps (a) through (e) with a green color filter and magenta toner,

g. repeating steps (a) and (b),

h. placing said photoreceptor and the undeveloped electrostatic image thereon in operative position relative to said support means and in registry with the developed image thereon,

i. adjusting the application of said electrostatic transfer charge to effect the removal from said support means of some of said magenta toner.

4. A xerographic color process including the steps of:

. a. charging a xerographic photoreceptor with electrostatic charge,

b. projecting an electrostatic image of an original object onto said photoreceptor by exposure through a red color filter,

c. developing said electrostatic image by application thereto of cyan toner,

d. applying an electrostatic transfer charge relative to a final support means and said photoreceptor,

e. transferring said developed image to said support means,

f. repeating steps (a) through (e) with a green color filter and magenta toner,

g. repeating-steps (a) through (e) with a blue color filter and yellow toner,

h. repeating step (a), I

i. projecting an electrostatic image of said original object onto said photoreceptor by exposure through said green color filter,

j. placing said photoreceptor and the undeveloped electrostatic image thereon in operative position relative to-said support means and in registry with the developed image thereon,

k. adjusting the application of said electrostatic transfer charge to effect the removal from said support means of some of said yellow toner. 

1. A xerographic color process including the steps of: charging a xerographic photoreceptor with electrostatic charge, projecting an electrostatic image of an original object onto said photoreceptor by successive exposures through color filters, developing said electrostatic image by application thereto of successive toners in color complementary to the respective filters, applying an electrostatic transfer charge relative to a final image support means and said photoreceptor, transferring said developed image to said support means, repeating said charging step, repeating a projection step through a color filter to recreate an electrostatic image of the original on the photoreceptor, placing said photoreceptor and the undeveloped electrostatic image thereon in operative position relative to said support means and in registry with the developed image thereon, adjusting the application of said electrostatic transfer charge to effect the removal from said support means of some of the toner material.
 2. A xerographic color process as defined in claim 1 wherein said color filters are red, green, and blue and said complementary toners are cyan, magenta, and yellow, respectively.
 3. A xerographic color process including the steps of: a. charging a xerographic photoreceptor with electrostatic charge, b. projecting an electrostatic image of an original object onto said photoreceptor by exposure through a red color filter, c. developing said electrostatic image by application thereto of cyan toner, d. applying an electrostatic transfer charge relative to a final support means and said photoreceptor, e. transferring said developed image to said support means, f. repeating steps (a) through (e) with a green color filter and magenta toner, g. repeating steps (a) and (b), h. placing said photoreceptor and the undeveloped electrostatic image thereon in operative position relative to said support means and in registry with the developed image thereon, i. adjusting the application of said electrostatic transfer charge to effect the removal from said support means of some of said magenta toner.
 4. A xerographic color process including the steps of: a. charging a xerographic photoreceptor with electrostatic charge, b. projecting an electrostatic image of an original object onto said photoreceptor by exposure through a red color filter, c. developing said electrostatic image by application thereto of cyan toner, d. applying an electrostatic transfer charge relative to a final support means and said photoreceptor, e. transferring said developed image to said support means, f. repeating steps (a) through (e) with a green color filter and magenta toner, g. repeating steps (a) through (e) with a blue color filter and yellow toner, h. repeating step (a), i. projecting an electrostatic image of said original object onto said photoreceptor by exposure through said green color filter, j. placing said photoreceptor and the undeveloped electrostatic image thereon in operative pOsition relative to said support means and in registry with the developed image thereon, k. adjusting the application of said electrostatic transfer charge to effect the removal from said support means of some of said yellow toner. 